The purpose of this study is to examine the exposure to certain chemicals, prior to or at the time of conception, that may play a role in damaging maternal or paternal germ cells and increase the risk of childhood leukemia. The risk of developing leukemia may involve a number of genes which confer susceptibility to the development of malignancy, especially in the immune system. One goal of this extensive project is to determine the relationship between HLA phenotype and the development of ALL.